The earlier levels of Twin Fighter from 20 to 26 are extraordinarily difficult. You're presented with a large array of skills and little guidance and synergy with most other classes, including Summoners which they were introduced with.
Once you can access F6 you should have a better flow of the Fighter skills. Primarily you should be using the skills that hit an enemy and slightly knock them back, which then allows you time to use other skills like Non Stop Kicking which will be a good DPS skill for a while, as well as a launch skill with Tornado.
You'll also have several filler skills like Spin Bomb and Spirit Spear. You'll want the multi hit skills like Sliding Kick which is also great DPS. Eventually you can get Rolling Grable which is absolutely wonderful for bosses and chunks of monsters. It can also combo into Non Stop Kicking. All of the main character's skills aren't too great so you won't really have to do much with both. Ring from Hell will function as your primary F7ing skill.
Once you've job changed to Mirage, you reach some of the harder hitting skills like Rising Drop Kick and Infinite Kicking--ditch Non Stop kicking for this. Rising Drop Kick ends up being your best mobbing, bossing and overall DPS skill in PvE. Some players find Stamper to be a decent DPS skill as well. In my experience the Fusion chain is mostly PvP oriented and I do not have any of it besides the actual Fusion skill for recalling the Twin for strategic use of skills. Here's a version of my build broken down into chunks of level:
Lv. 30
http://irowiki.org/~...000000000000000
Lv. 50
http://irowiki.org/~...000000000343301
Lv. 80
http://irowiki.org/~...500030000353531
Keep in mind the Passives Heavy Blow and Iron Fist Training have inaccurate descriptions. Heavy Blow adds Critical Rate and Iron Fist Training adds Critical Damage.